This invention relates generally to air conditioning systems and, more particularly, to a method and apparatus for varying the effective orifice of an expansion device in a variable mass flow refrigeration system.
In a typical refrigeration cycle, the refrigerant flow to the evaporator coil is metered by way of an expansion device which restricts the flow of refrigerant in a controlled manner to thereby control the amount of superheat in the system. Capillary tubes and thermal expansion valves (TXV's) are the most common types of expansion devices used in refrigeration systems. Capillary tubes are of fixed dimensions selected to provide the desired operational characteristics for a particular system, whereas a TXV is a variable flow device which is automatically modulated in response to superheat conditions to thereby vary the refrigerant flow in such a way as to obtain the desired superheat conditions.
Another type of expansion device used in refrigeration systems is a so-called "accurater", shown and described in U.S. Pat. No. 3,992,898, assigned to the assignee of the present invention. This patent is hereby incorporated herein by reference. An attractive feature of this device is the ability to select and change pistons so that the orifice size can be chosen to match the desired flow characteristics of a particular system. Once installed, the piston, like a capillary tube, has a fixed dimension. But unlike a capillary tube, the accurater can be easily and economically adapted for use with any of a number of system load conditions by changing the piston size.
Heretofore, refrigeration systems have primarily been operated at a single constant speed. That is, the compressor was either turned off, in which case the system was not in operation, or turned on to run at a constant speed. The expansion devices as mentioned hereinabove, were therefore chosen to be compatible with the flow characteristics of that operational speed. In some cases, a two-speed compressor was used to provide two discreet levels of refrigerant flow conditions. These two flow volumes have traditionally been accommodated by a TXV which automatically provided a smaller expansion orifice when operating at the lower speed.
In an effort to optimize the operating efficiency of a refrigeration system, the applicants have applied a variable speed motor to drive a compressor. One of the characteristics of such a variable speed system is that of operating within a continuous and infinite range of speeds rather than at one or two discreet levels. Although a TXV may be responsive to vary its orifice over a range of speeds, it was not designed to operate in this manner. Accordingly, its accuracy and responsiveness have been found to be unsuitable for use in such a variable speed system. Further, the operating range of a typical TXV is not sufficient for use in such a system where the compressor is driven by a variable speed motor.
It is therefore an object of the present invention to provide an improved expansion device for a variable speed air conditioning system.
Another object of the present invention is the provision in an air conditioning system for metering the flow of refrigerant over a continuous range of compressor speeds.
Yet another object of the present invention is the provision in a variable speed air conditioning system for accurately and responsively varying the refrigerant flow with the speed.
Still another object of the present invention is the provision in a variable speed air conditioning system for an expansion device which is economical to manufacture and extremely functional in use.
These objects and other features and advantages become more readily apparent upon reference to the following description when taken in conjunction with the appended drawings.